When a blade of a cutting tool is chipped during the machining of a workpiece in turning, a torque of a motor which moves the cutting tool and a torque of a spindle motor which moves the workpiece are changed. A tool abnormality determination system detects chipping based on the torque changes. That is, the tool abnormality determination system compares an actual torque change to a threshold value for determining abnormality and, when the actual torque exceeds the threshold value, determines that chipping occurs.
However, when the same type of workpiece is machined, a torque value or a torque fluctuation range changes depending on a change in material lot, a machining part, and a traveling direction of a tool. Therefore, when the threshold value is fixed, the possibility of erroneous determination is increased.
PTL 1 discloses a method of monitoring a machining load, the method including: obtaining sampling data of a torque of a motor through multiple trial cutting operations; obtaining reference data and variance from the sampling data; and setting a threshold value according to variations in the sampling data. According to the method of monitoring a machining load disclosed in PTL 1, a threshold value for a machining part having a large torque variation can be set to be large. Conversely, a threshold value for a machining part having a small torque variation can be set to be small.